1. Field of the Invention
The invention relates to an interconnection (contact) element suitable for effective connections between electronic components.
2. Description of Related Art
Interconnection or contact elements may be used to connect devices of an electronic component or one electronic component to another electronic component. For example, an interconnection element may be used to connect two circuits of an integrated circuit chip or to connect an application specific integrated circuit (ASIC). Interconnection elements may also be used to connect the integrated circuit chip to a chip package suitable for mounting on a printed circuit board (PCB) of a computer or other electronic device or to connect the integrated circuit chip directly to the PCB. Interconnection elements may further be used to connect the integrated circuit chip to a test device such as a probe card assembly or other substrate to test the chip.
Generally, interconnection or contact elements between electronic components can be classified into at least the two broad categories of “relatively permanent” and “readily demountable.”
An example of a “relatively permanent” contact element is a wire bond. Once two electronic components are connected to one another by a bonding of an interconnection element to each electronic component, a process of unbending must be used to separate the components. A wire bond interconnection element, such as between an integrated circuit chip or die and inner leads of a chip or package (or inner ends of lead frame fingers) typically utilizes a “relatively permanent” interconnection element.
One example of a “readily demountable” interconnection element is the interconnection element between rigid pins of one electronic component received by resilient socket elements of another electronic component, for example, a spring-loaded LGA socket or a zero-insertion force socket. A second type of a “readily demountable” interconnection element is an interconnection element that itself is resilient or spring-like or mounted in or on a spring or resilient medium. An example of such an interconnection element is a tungsten needle or a micro-spring contact of a probe card component such as described in commonly assigned U.S. Pat. No. 5,974,662, titled “Method of Planarizing Tips of Probe Elements of a Probe Card Assembly,” issued Nov. 2, 1999 (FFI-P06). The interconnection element of a probe card component is typically intended to effect a temporary pressure connection between an electronic component to which the interconnection element is mounted and terminals of a second electronic component, such as a semiconductor device under test.
With regard to spring interconnection elements, generally, a minimum contact force is desired to effect reliable pressure contact to an electronic component (e.g., to terminals of an electronic component). For example, a contact (load) force of approximately 15 grams (including as little as 2 grams or less and as much as 150 grams or more, per terminal) may be desired to effect a reliable electrical pressure connection to a terminal of an electronic component.
A second factor of interest with regard to spring interconnection elements is the shape and metallurgy of the portion of the interconnection element making pressure connection to the terminal of the electronic component. With respect to the tungsten needle as a spring interconnection element, for example, the contact end is limited by the metallurgy of the element (i.e., tungsten) and, as the tungsten needle becomes smaller in diameter, it becomes commensurately more difficult to control or establish a desired shape at the contact end.
In certain instances, interconnection elements themselves are not resilient, but rather are supported by a resilient membrane. Membrane probes exemplify this situation, where a plurality of microbumps are disposed on a resilient membrane. Again, the technology required to manufacture such contact elements limits the design choices for the shape and metallurgy of the contact portion of the contact elements.
Commonly-assigned U.S. patent application Ser. No. 08/152,812, filed Nov. 16, 1993 (now U.S. Pat. No. 5,476,211, issued Dec. 19, 1995) (FFI-P01), discloses methods for making spring interconnection elements. In a preferred embodiment, these spring interconnection elements, which are particularly useful for micro-electronic applications, involve mounting an end of a flexible elongate element (e.g., wire “stem” or “skeleton,” sometimes “core”) to a terminal on an electronic component, and coating the flexible element and adjacent surface of the terminal with a “shell” of one or more materials. One of skill in the art can select a combination of thickness, yield strength, and elastic modulus of the flexible element and shell materials to provide satisfactory force-to-deflection characteristics of the resulting spring interconnection elements. Exemplary materials for the core element include gold. Exemplary materials for the coating include nickel and its alloys. The resulting spring interconnection element is suitably used to effect pressure, or demountable, interconnections between two or more electronic components, including semiconductor devices.
As electronic components get increasingly smaller and the spacing between terminals on the electronic components get increasingly tighter (the pitch gets increasingly finer), it becomes increasingly more difficult to fabricate interconnections suitable for making electrical connection to terminals of an electronic component. Co-pending and commonly-assigned U.S. patent application Ser. No. 08/802,054, filed Feb. 18, 1997, titled “Microelectronic Contact Structure, and Method of Making Same” (FFI-P34) (incorporated herein in full by reference), and corresponding PCT application published Nov. 27, 1997 as WO97/44676, disclose a method of making spring interconnection elements through lithographic techniques. In one embodiment, that application discloses forming a spring interconnection element (including a spring interconnection element that is a cantilever beam) on a sacrificial substrate and then transferring and mounting the interconnection element to a terminal on an electronic component. In that disclosure, the spring interconnection element is formed in the substrate itself through etching techniques. In co-pending, commonly-assigned U.S. patent application Ser. No. 08/852,152, titled “Microelectronic Spring Contact Elements” (FFI-P35), spring interconnection elements are formed on a substrate, including a substrate that is an electronic component, by depositing and patterning a plurality of masking layers to form an opening corresponding to a shape embodied for the spring interconnection element, depositing conductive material in the opening made by the patterned masking layers, and removing the masking layer to form the free-standing spring interconnection element.
Co-pending and commonly-assigned U.S. patent application Ser. No. 09/023,859, titled “Microelectronic Contact Structures and Methods of Making Same” (FFI-P047) describes an interconnection element having a base end portion (post component), a body portion (beam component) and a contact end portion (tip component) and methods of separately forming each portion and joining the post portion together as desired on an electronic component.
U.S. Pat. No. 5,613,861 (and its counterpart divisional U.S. Pat. No. 5,848,685) issued to Smith et al. disclose photolithographically patterned spring interconnection elements formed on a substrate with a body having an inherent stress gradient formed of a resilient (e.g., elastic) material such as chrome-molybdenum alloy or a nickel-zirconium alloy. The stress gradient causes an end of the body to bend away from the substrate in the shape of an arc when the end is freed from the substrate.
In order to achieve the desired shape of the body, Smith et al. must limit the thickness of the interconnection element described in U.S. Pat. No. 5,613,861. A limit on the thickness of the interconnection element limits the spring constant, k, of the interconnection element (k increases as thickness increases) particularly in state-of-the-art interconnection element arrays where the dimensions (e.g., length and width) of individual interconnection arrays are reduced to accommodate a corresponding increase in contact pad or terminal density. A reduction of the spring constant generally reduces the amount of load or force, F, that may be applied to resilient interconnection elements for a given deflection, x (k=F/x). Thus, such interconnection elements generally sustain at best a moderate contact force, which may not be enough to effect reliable pressure contact to an electronic component.
What is needed is a resilient interconnection element and a method of improving the resiliency of an interconnection element, particularly interconnection elements that are suitable for present fine-pitch electrical connections and that is/are scalable for future technologies. Also needed are improved methods of making resilient interconnection elements, particularly methods that are repeatable, consistent, and inexpensive.